I have an e5-2696v3 on X99. That one turbos up to 3.8, but very, very rarely: It really is an 2.8GHz all-core CPU with 18 cores instead of 14 and a slightly higher TDP.
Real overclocking is out of the question, because of the multiplier locks.
BCLK overclocking can be done: I have used a 104.8 clock to get mine to 4GHz single core turbo, but went back to normal on a hot day, when it got stuck booting, just because it really wasn't worth on a system that is used productively.
There seems to be an exploitable bug in the original microcode on Haswell Xeons, which allows you to effectively run all cores at max Turbo speed, bypassing the PL1 and PL2 limits. And there are tools and hacks that describe what you have to do in order to back-patch your microcode in the BIOS and keep Windows from updating the microcode on the fly etc. obviously also reverting all the Meltdown/Specte fixes as well as the TSX instruction block...
Patching your CPU microcode and your BIOS isn't for the faint hearted, plenty of things to go wrong. And unless you have a motherboard with A-B BIOS capacities, I woudn't dare go near it. I have dual BIOS but still decided I wasn't about to risk killing one of my major workhorses, when so few actual workloads would have critical benefits.
Because after you have achieved re-inserting the original microcode bug, there is simply the risk that your CPU will fail to work with 3.1 all core turbo, because it wound up in this bin for a reason as a result of validation.
If you need high-frequency and fewer cores, I'd say simply get another CPU.
Search on eBay, sometimes you get lucky. I got my e5-2696v3 for $700, which is probably the best CPU deal I ever made.
Still not watering at the mouth over all these i9 or Ryzen 3, because my Cinebench still trumps them all and for two years running.